Laser induced breakdown spectroscopy (LIBS) is a technology that is based on a process called laser ablation. The basic idea of laser ablation is relatively straightforward. A high powered pulsed laser, capable of achieving a very high irradiance, is focused, using a system of mirrors and lenses onto a very tiny spot on a target material. When the laser energy exceeds the ablation threshold energy of the material, chemical bonds are broken and the material is fractured into energetic fragments. These fragments include a mixture of neutral atoms, molecules and ions inducing plasma. During plasma formation, electrons interact and subsequently, within microseconds, recombine with ions to release energy across a broad spectral range, most importantly for LIBS between 200 to 980 nanometers. Basic LIBS equipment, which are commercially available include a pulsed laser, capable of inducing sufficient irradiance to ablate a sample target, a light collection system, consisting of lenses mirrors or a fiber optic that collects the plasma light and transports the light to a detection system, a detection system, such as spectrograph and a detector, such as a charged couple detector, to record the light and computer and electronics to gate the detector, fire the laser and store the spectra.
The light generated by the plasma is characteristic of the chemical makeup of the ablated material. This light (emission) can be quantified by collecting it and generating a spectral image that identifies the emission wavelengths and respective intensities in a spectrometer. This spectral image can be visually and mathematically recorded in a computer. Once stored it is possible to make use of spectral patterns to quantify elemental concentrations present in the sample. For example, each element emits a characteristic set of discrete wavelengths according to its electronic structure. By observing these wavelengths and their respective emission intensities, the elemental composition of the sample can be determined. As such, LIBS is fundamentally similar to other traditional atomic emission spectroscopic methods; but unlike traditional methods lasers have the unique advantage of providing real-time “remote sensing” capability both in the laboratory and in particular in field applications, without special sample preparation.
During the past decade, the dissemination of information and descriptions of potential applications for this technology has appeared in numerous publications such as the Journal of Applied Spectroscopy, the Journal of Analytical Atomic Spectrometry, and the Journal of Applied Optics and Spectrochimica Acta. In addition, the topic has generated the publication of no less than four textbooks devoted to the subject, including Noll, R., Laser Induced Breakdown Spectroscopy, Springer-Verlag, Berlin Heidelberg 2012, Cremers, D. and L. J. Radziemski, Handbook of Laser-Induced Breakdown Spectroscopy, John Wiley, England 2006, Singh, J. P. and S. N. Thakur, Laser-Induced Breakdown Spectroscopy, Elsevier, Amsterdam 2007, and Mizolek, A. W and V. Palleschi and I. Schecter, Laser-Induced Breakdown Spectroscopy, Cambridge University Press, New York, 2006. These texts describe not only the fundamentals of the technology, but numerous potential applications for which the LIBS technology can be employed. Some of these applications include archaeological material analysis, toxic materials identification in liquids, explosive material detection, soil analysis, food contamination, geologic material and gemstone identification, and metal alloy detection. The National Aeronautics and Space Administration (NASA) is employing LIBS technology in space on the exploration vehicle, Curiosity that touched down on the Mars landscape on Aug. 13, 2012, to determine the mineralogy of soil and rock on the Martian landscape.
Despite the number of potential applications and its current use in space, LIBS technology has been slow to be deployed in bulk material commercial applications. Most of the current LIBS applications employ techniques that focus on precise targeting at a fixed focal point. Each time a new target is identified a new focal point is established and the laser is fired sometimes at 1-50 Hz to collect spectral information about the target. These systems are referred to herein as “shot to shot” systems. The goal of such systems is try to duplicate, to the extent possible, the conditions each and every time the laser fires at the sample target. In shot to shot systems, the sample population is relatively low and since only a very small area (typically less than 0.2 mm in diameter) is targeted, it is critical in such systems that the selected target location be representative of the sample. It is also important that the laser and optical collection and detection system operate at highly stable conditions to eliminate fluctuations in the spectral output. Shot to shot system stability is extremely important for LIBS because any change in operating parameters such as laser power, focal length, system temperature or environmental conditions could alter the properties of the plasma matrix and the resulting spectral output; and with few samples the spectral output could clearly misrepresent the target material. Such systems are most effective when the target material has a high degree of homogeneity from sample to sample.
In many industrial applications it is desirable to provide in-line, real time continuous or semi-continuous monitoring of bulk materials that display a significant degree of heterogeneity during the production process. The purpose is to provide improved quality control procedures. This requires monitoring systems that can continuously scan high volumes of materials, and data processing systems that incorporate analytical techniques that can integrate the data, in this case the spectral outputs, into information about the nature of the target material. Examples of industrial applications where the monitoring systems would be of great utility include the Portland and asphalt concrete production industries, and the cement, glass, steel, coal, mining and mineral aggregate production industries.
A current method for in-line bulk materials analysis, particularly in the cement, coal and mining industry is prompt gamma ray neutron activation analysis (PGNAA). In this technology the bulk material is bombarded with neutrons as it passes a neutron source, releasing gamma rays. Different gamma ray energy spectra are produced from different elements in the bulk materials. By processing these detected signals, the elements present in the bulk sample can be quantified. Similar systems are being researched and deployed using x-ray fluorescence (XRF),
PGNAA and XRF are further advanced than LIBS in their commercial deployment. Up until now, with the few exceptions outlined below, LIBS systems have not been used in bulk sampling in an industrial environment. This is because the heterogeneity of samples with respect to chemical properties, moisture content, dust particles and atmospheric conditions, such as humidity and temperature as well as gas composition interferes with the laser irradiation and spectra generated by the laser shot. In addition, rapid firing of a laser at a moving stream of bulk material will not always result in a constant laser to target focal length, introducing additional variation in the spectral output. The subject invention is designed to transform the use of LIBS from a “shot to shot” laboratory or a roving mobile system to an industrial monitoring system capable of providing quality control monitoring of bulk materials. Bulk materials used in the context of this invention are coarse granular materials or fine grained granular material or powdered materials. Examples of such materials include, mineral aggregates used in construction, mined ores, geologic rock, glass, cement, and coal. Such materials can be chemically heterogeneous, depending on the source, and tend to be processed in large quantities on the order of tons per hour. Determining the chemical, physical and mechanical nature of such materials, in real time, is difficult without the use of a continuous in-line monitoring system that can collect and process large quantities of spectral data to provide information on the nature of the material. Such information can be used as a quality control tool to reject or accept materials during processing.
There are at least four common features in bulk monitoring emission spectroscopic detection systems, regardless of the technology employed. The first is that a source of energy must be available to target the bulk materials. The second is that the bulk material or subsamples of the bulk materials must be transported to or passed near the source to expose the bulk material to the energy source. The third is that the atomic emission released from the targeted material must be captured in a suitable detector and the fourth is that the information detected must be processed and translated into quantifiable information that is suitable for the quality control objectives. To integrate all four features into an operating system, a suitable material transport and targeting and environmental control system must be available to practically adapt the process to the specific technology (PGNAA, XRF, or LIBS) being employed.
The following summaries of prior art focus on the integrated process strategies that have been reported for PGNAA, XRF and LIBS systems:
Atwell (U.S. Pat. No. 4,582,992) describes a real time PGNAA bulk material analyzer in a self-contained sealed air conditioned housing, where bulk material is channeled through a vertical three foot chute. Atwell describes chute deployment at the end of a conveyor belt and the transport of material vertically down through the chute to a lower conveyor. The radiation sources and the detectors are symmetrically located on opposite sides of the chute to target the materials with neutrons and detect the gamma ray emissions. Such systems must be extremely well sealed to prevent any release of gamma radiation. Vourvopoulos (U.S. Pat. No. 6,438,189 B1) describes a system in which the inventor introduces an apparatus, which appears to be a bifurcated hopper that is used to direct a portion of a target material (particularly coal or cement) from the main chute to an alternative or secondary chute. The main chute is surrounded by a neutron generator and a system of gamma ray and neutron detectors. The alternative chute receives gamma emissions resulting from reactions in the main chute, and generates secondary emissions that are also recorded. The combination the main and secondary spectral outputs are analyzed to yield the elemental composition of the target material. Lingren (US Patent 2003/0225531 A1) describes the use of a PGNAA source and gamma ray detectors integrated into a specially designed cylindrical housing with an inner and outer chamber to protect the release of radiation. Osucha (U.S. Pat. No. 7,006,919 B2) describes another similar method of irradiating a flowing material on a conveyor with neutrons as part of a PGNAA detection process, but Osucha's emphasis is the development of a library of spectra to match the spectral output of unknown samples with unknown levels of impurities to the spectra of samples with known levels of impurities. The objective is to predict the level of impurities in the unknown samples.
Murray (U.S. Pat. No. 4,428,902) describes a system in which high energy x-rays from an electron accelerator induce the emission of gamma rays from a target material, specifically coal, to measure the oxygen and sulfur content of the targeted coal samples. In Murray's embodiment, the x-rays from a linear accelerator are directed from a location above the conveyor belt carrying the coal, irradiating the coal with a subsequent release of gamma rays. The gamma radiation is detected by a suitable gamma ray spectrometer located under the conveyor to determine the quality of the coal. Again, such systems must be extremely well shielded to protect exposure to dangerous gamma radiation.
Connolly (U.S. Pat. No. 5,818,899) describes a system in which an x-ray transmitter and an x-ray fluorescent detector are integrated into a pulverized coal (powder) supply line to determine the chemical composition of the coal. Connolly's invention provides for a recessed chamber within the coal feed line (tube) that is designed to collect discrete pulverized coal samples at predefined intervals. The samples are subsequently bombarded through a transparent window in the recessed chamber with x-rays. An adjacent probe receives fluorescence resulting from the x-ray bombardment and transmits the data for subsequent analysis. Pressurized air discharges the powdered coal back into the coal feed line after the analysis, providing room for the next sample to be collected. Mound (US Patent US 2007/0263212 A1) describes a non-hazardous bulk material analyzer system that makes use of white light (not x-rays or gamma radiation or laser light) to scan target materials (focusing on cement) from a source located above a conveyor. Mound also describes the use of pre-calibrated spectra that are generated using chemometric techniques to provide a stored library of spectra to which unknown samples can be compared. Mound does not describe the chemometric techniques and provides no specifications on specific sources of white light, emission detectors, nor does he address any of the myriad of interfering factors, such as dust or focal length control, or environmental control that would be needed to practically apply such a system in an industrial setting. Sommer (U.S. Pat. No. 7,616,733 B2) describes the use of an XRF system deployed on a conveyor belt that can be sequentially irradiated with x-rays, along the length of the belt to identify and, sort the target material, particularly ferrous and nonferrous materials from automobile shredding operations.
There are numerous patents that have been filed that focus on “shot to shot” (laboratory based in most cases) LIBS systems incorporating specific applications or equipment innovations or arrangements. It is much easier to control factors that define the plasma matrix in controlled shot to shot systems where the target is placed at a fixed focal point and where environmental conditions are better controlled than an industrial processing line. Integrating LIBS into an in-line processing system, requires a LIBS process that can sample and process the target in a way that manages the environmental factors and processes the data generated.
The first US Patent to express the use of LIBS system for monitoring bulk materials in an in-line process is believed to be Potzschke (U.S. Pat. No. 5,042,947). Potzschke describes a method for using a LIBS system on a conveyor belt to identify the composition of scrap metal, but provides little detail on the physical nature of such a system and how it would target the material. Graft (U.S. Pat. No. 6,753,957 B1) describes an in-line bulk monitoring LIBS system designed to target bulk materials, particularly phosphate rock being conveyed on a moving belt. To resolve spectral resolution problems, Graft focuses his invention on analytical techniques through the development of spectral elemental signature ratios to determine whether a specific element is present or not in the sample. In a subsequent publication (Gat M.; Sapir-Sofer, I.; Modiano, H.; Stana, R. Laser induced breakdown spectroscopy for bulk minerals online analyses, Spectrochimica Acta Part B: Atomic Spectroscopy, Volume 62, Issue 12, December 2007, p. 1496-1503) Gaft (Note: Gaft is believed to be the Graft of U.S. Pat. No. 6,753,957B1) reported on the development of a LIBS in-line conveyor and field prototype unit for phosphate rock ore analysis and the ash content evaluation of coal. He describes a LIBS system located over a conveyor belt carrying phosphate ores containing sealed panels, shock absorbers and air conditioning to enable outdoor operation.
Laser targeting of bulk materials is limited by the harsh, dust-laden and vibrating environments associated with industrial processing operations. Such environments can interfere with laser targeting and the laser to target focal length, which can alter poorly understood matrix effects associated with the ablation process. Non-uniform ablation conditions could significantly affect the intensities and spectral emission of the process. In addition, the presence of dust and the atmospheric gas composition, including the humidity, in the vicinity of the laser induced ablation, and moisture in the samples could interfere with the laser energy reaching the targeted material and the nature of plasma formation and ultimately the light emitted during the ablation process. The laser and optical system must be segregated from the dust particles to prevent a buildup of dust in the system and corresponding operational difficulties. Finally, safety issues in a bulk material production environment demands that eye or skin exposure to operating personnel be avoided. Open and exposed targeting of materials in such an environment would be problematic.
The subject invention is designed to transform the use of LIBS from a “shot to shot” laboratory or a roving mobile system to an industrial monitoring system capable of providing quality control monitoring of bulk materials. Bulk materials used in the context of this patent are high volume coarse granular materials or fine grained granular material or powdered materials
LIBS spectra are well suited as input data for use in the development of chemometric multivariate statistical models. Multivariate statistical models are capable of quantifying patterns in the spectra data, which is generated by the laser ablation process. Such models are most effectively applied by establishing a spectral pattern model associated with a known sample, referred to as the calibration sample, and using this defined calibration model to discriminate and identify similar or dissimilar patterns in unknown samples. This technique of pattern matching is similar to fingerprinting. As part of the subject invention, as will become apparent, the inventor makes use of these models to assist in processing and interpreting an extremely large output of spectral data generated as a result of the sampling and targeting system discussed in this patent. It is this Sampling and Targeting System, referred to as the SLT, which provides the means to process bulk materials through a specially conceived laser-optics system designed to procure data necessary for characterizing the bulk materials.